As known by the public, the electromagnet is a low-voltage electric apparatus for converting electrical energy to mechanical energy, and is widely applied in industrial fields and various products. When the exciting coil of its excitation system is powered to form a magnetic field incorporating with the iron core, the magnetic field attraction force attracts the armature with mechanical load towards the coil and the iron core so that the electromagnet starts to work. When the armature stops moving and turns to the holding state, the effect of the current is only to maintain the rated holding force between the iron core and the armature.
The conventional electromagnets can be divided into two kinds, i.e., AC and DC. Based on the principle of constant magnetic flux linkage, the AC electromagnet has the advantages of increasing automatically the initiating power and decreasing automatically the holding power. Therefore, it has attained very wide applications in industries. However, the AC electromagnet has the disadvantages of extremely low power factor, iron loss (eddy-current and magnetic hysteresis loss), magnetic separation ring loss, pulsing force loss, hum noise, and burning of the coil due to the armature overload. Based on the principle of constant magnetic potential, the DC electromagnet has the advantages of high power factor (without any back electromotive force in holding state), no iron loss in the holding state, no magnetic separation ring, no noise, no burning of the coil due to armature overload. The DC electromagnet, however, has the disadvantages of a very steep characteristic curve of attraction force, and is incapable of increasing automatically the initiation power and decreasing automatically the holding power. It can only be used in operating modes of very low initiation load and very short working stroke. Thus, its applications are far less than that of the AC electromagnets.
In order to overcome the disadvantages of both AC and DC electromagnets and combine the advantages of both, the variable magnetic potential DC electromagnet technology has been proposed. There are two approaches for this technology, one is the double coil approach and the other is the double power source approach.
A double exciting coil technical approach was disclosed by Japanese patent laid open bulletin 59-175709 on Oct. 4, 1984, wherein the holding coil is shorted out by the normal close contacts at the DC initiation of the electromagnet, and the initiation coil and the holding coil are connected in series by the separation of the normal close contacts at the termination of initiation, thereby turning into the holding state. Obviously, a higher initiation current can be obtained when the initiation coil works individually, and a lower holding current can be obtained when the two coils work in series. Although this approach that has the advantages of both AC and DC electromagnets has overcome partially the disadvantages of the DC electromagnet, and has widened its application scope, as well as is energy saving, there are still some disadvantages, such as the complicated production process of the double coil, the waste of copper material, the limit of the holding current due to the use of the same voltage power source, the generation of electric arc at the separation of the mechanical contacts, and the burning of the coil due to a false separation of the contacts.
The double power source approach may compensate the disadvantages of the double coil approach. The double power source approach uses only a single exciting coil, while it is powered separately by two DC sources at initiation and in holding state. This brings a great convenience to the high magnetic potential initiation and low magnetic potential holding, since the voltage difference between the two sources could be very large. This approach has been disclosed in an article of "Changing AC braking electromagnet to DC operating" in a Chinese publication "Low Voltage Apparatus" vol. II, 1985 and a Chinese patent application (publication No. CN1038543A). The disadvantages of the above mentioned references are: not only the AC component of one-half period rectification or one-half period conduction through phase shift at its initiation is large, but also the high response initiation and large stroke operation cannot be achieved due to the limited raising of the magnetic potential at the initiation. It is likely to cause mechanical impact at the instant of attracting together, since the electromagnet is continuously powered by the initiation power source in the whole procedure of the movement of the armature; the thyristor as a one-half period rectification device for the initiation power source in the above mentioned article of "Low Voltage Apparatus" is triggered in the manner of unidirectional voltage bypass triggering, which is seriously affected by the phase at the instant of closing the power supply switch of the electromagnet for initiation. Thus, the initiation time may be delayed. The problem of delayed initiation time also exists in the above mentioned Chinese patent application when the voltage at the instant of closing the switch is in the negative half period.
In addition, the holding power source commonly adopts transformer and capacitor for voltage reduction in the double power source energy saving technique of the existing electromagnets. Since the non-load current of the transformer and the exciting current are of the same magnitude for medium and small holding force electromagnets, the voltage reduction by the transformer is only appropriate to the electromagnets of large holding force, while the voltage reduction by the capacitor is only appropriate to the small holding force electromagnets. The sizes of the transformers and capacitors are too bulky when they are used for the large and medium holding force electromagnets, that is to say, the existing double power source techniques are difficult to satisfy the requirements of different loads. Further, the voltage reduction by the transformer and capacitor has the drawback of being difficult to be modularized.
The variable magnetic potential DC electromagnet saves energy mainly by improving its power factor, i.e., by reducing reactive power loss. As proved by experiments, the demagnetizing air gap kept by the existing electromagnets consumes approximately 60%-95% of the active power of the electromagnets. Therefore, the advantage of energy saving by providing extremely low holding voltage of the double power source cannot be exploited sufficiently, if the mechanical structure of the electromagnet per se cannot reduce the active power loss efficiently. In addition, since the holding force between the iron core and the armature is reduced by the high magnetic resistance produced by the demagnetizing air gap, the coil, iron core, and armature have to be designed to be bulky in order to maintain their rated holding force, thereby resulting in the waste of large amounts of copper and iron materials.